52 



RUSSELL J. BEERS 



XO GROWTH 

 Zone 1 — 



RESTRICTED GROWTH 

 Zone 2 



NORMAL GROWTH 

 Zone 3 



Fig. 1. Influence of Moisture on Spore Germination and Growth [Dia- 

 gram aticl. 



Dr. Beers has reported that this point is believed to exist in the region of 

 approximately 95 percent relative humidity for many bacterial spores and 

 it is perhaps as low as 70 percent for molds. One research goal of the food 

 bacteriologist is to locate and clearly define the limits of Zone 2 for foods and, 

 particularly, the variation in the position of point B. It seems, at present, 

 that this geometry may be shifted somewhat in accordance with other com- 

 plex parameters. For a given species at least two factors may now be recog- 

 nized on this issue, namely, (a) the pH of the medium, and (b) the age of 

 the spores. Undoubtedly there are others. 



Halvorson (1949) has employed the term "moisture activity" to refer in 

 the thermodynamic sense to the effective concentration of water in the sub- 

 strate. As determined by vapor pressure techniques, this activity is equivalent 

 to a relative humidity measurement of the spores' environment at equilibrium 

 conditions. For a given food one may expect to find a limiting moisture 

 activity preventing spore germination and growth. An important bacterio- 

 logical distinction should be made between the moisture activity of a sub- 

 stance and its total water content. Since the latter value includes some 

 water which may be regarded as unavailable to the organism, it is necessary 

 to know the relation between the total moisture content and the activity of 

 the water in the substrate under consideration. Only recently have some of 

 these relationships been worked out. Two illustrations are shown in Table I. 

 It will be noted that for these difl^erent substrates the relationships differ. 

 Whether the limiting moisture activities for the different foods are exactly 

 the same, however, has not yet been proven. Some studies related to the 

 moisture limitations on spore germination have been reported for canned 

 bread (Halvorson, 1949; Kadavy and Dack, 1951; Ulrich and Halvorson. 

 1948-49; Wagenaar and Dack, 1954), beef liver paste (Williams, 1950 and 

 1953), canned cheese spreads (Wagenaar and Dack, 1955), and for such de- 

 hydrated meats as beef and pork (Segalove and Dack, 1951). Data on the 

 germination of Clostridium botulinum spores for the two products shown 

 in Table I will illustrate this point. In the experimental development of 



